In many cases the cost of installing an “on line” analyzer for a natural gas or other process stream cannot be justified. In such cases, a “spot” sample is taken periodically, or a composite sample is taken over a period of time. A “spot sample” generally consists of extracting a sample of the gas at a “spot”, or single point in time.
A composite sample is generally taken by a sampling apparatus (composite sampler) which extracts a small volume (bite) of gas sample periodically, which is collected in a sample cylinder. A typical composite sampler will control the sampling interval based on time or flow volume. Flow volume information, or electric pulse, is usually provided by an external flow computer. A typical composite collection period is 30 days. There are two basic types of sample cylinders utilized for spot and composite sampling. They are the constant volume and constant pressure types of sample cylinder. Constant pressure types of sample cylinders are of the floating piston or bladder (bag) type.
Gas Processors Association Standard Publication 2166 entitled “Obtaining Natural Gas Samples for Analysis by Gas Chromatography” details several spot sampling methods in its 1986 revision and its draft. The main thrust of the GPA 2166 standard deals with methods for purging the sample cylinder, dealing with entrained liquid using the GPA or other type separator, and prevention of condensation of gas components. The draft copy addresses major errors in the 1986 revision. In particular, it dwells on the use of external separator to remove liquids at other than the source gas conditions of pressure and temperature.
American Petroleum Institute Manual of Petroleum Measurement Standards, Chapter 14, Section 1, revised in 2001, the contents of which are incorporated herein by reference, addresses spot and composite sampling issues. It is critical of the GPA 2166-1986 standard, in particular, the use of an external separator such as the GPA separator, for removing entrained liquids (refer to 14.1.8.7). The API 14.1 standard-rev 2001, is based on research work done by Southwest Research Institute.
The hydrocarbon dew point temperature (HCDPT) and its impact on the sampling of natural gas is the main focus of the API 14.1 standard. This is evident in the opening paragraph “14.1.1 Introduction”. Refer to 14.1.6, here the standard calls out issues which need to be addressed when sampling natural gas. Issues include ambient temperature condition and phase-change characteristics. In 14.1.6.6, it makes clear that no part of the spot or composite sampling system should be allowed to fall below the HCDPT, otherwise biased analytical results and non-representative samples are likely to result. Maintaining the sample system at, or above, the HCDPT is recommended.
In 14.1.6.6.4 (Sample containers), it states that the cylinder temperature must be kept above the HCDPT. Note that the author is a member of the API 14.1 working group responsible for the writing the standard. The reason that the recommendation is for the cylinder temperature to be “above” as opposed to “equal to or above” the HCDPT is due to the uncertainty of measuring the HCDPT. It is well known, from a thermodynamic standpoint, that maintaining a gas at the HCDPT will not yield condensation.
Referencing the 14.1.6.6.6 composite sampling systems in the above publication, in short, it recommends that these systems, including the containers, (a direct quote) be maintained above the HCDPT to ensure that the composite sample is representative of the flowing gas stream. Similar references are made in 14.1.13.1.1.